We have examined the effect of FoxP3 on class I gene expression both in vivo and in cell culture. In cell culture assays, FoxP3 has a dramatic effect on MHC class I promoter activity. However, the effect is cell type dependent. Thus, FoxP3 enhances promoter activity in Jurkat T cells but inhibits it in Hela epithelial cells. Although only a single canonical FoxP3 binding site occurs within the promoter, multiple response elements have been mapped in both cell types. Intriguingly, the single FoxP3 binding site is located within the IRE of the promoter, such that mutation of the IRE abrogates the FoxP3 response. Furthermore, although FoxP3 is known to interact with RUNX1 and RUNX1 enhances class I promoter activity, the two factors function independently. Most intriguingly and consistent with the cell culture findings, class I expression in Tregs in vivo is elevated relative to the CD4+CD25- T cell subset. The functional significance of this over-expression, if any, is being pursued.